The present invention relates to a method and apparatus for manufacturing a semiconductor physical quantity sensor that senses physical quantities such pressure, acceleration and angular velocity.
FIGS. 7(a)-(b) show the essential structure of a conventional semiconductor physical quantity sensor. FIG. 7(a) is a plan view, and FIG. 7(b) is a sectional view cut along line Yxe2x80x94Y in FIG. 7(a).
As shown in FIGS. 7(a)-(b), an SOI substrate 100 consists of a silicon substrate 1, an oxide film 2 and a silicon layer 3 (a single crystal layer or a polysilicon layer). A semiconductor physical quantity sensor is formed in the silicon layer 3, which is the third layer on the SOI substrate 100. The semiconductor physical quantity sensor consists of a sensing element 103, a digital adjustment circuit 104, an analog amplifier circuit 105, an input/output terminal 106 and a digital adjustment terminal 107. The sensing element 103 is warped as indicated by an arrow in FIG. 7(b) by pressure, acceleration and angular velocity. The semiconductor physical quantity sensor sensing physical quantities such as pressure, acceleration and angular velocity by amplifying electric signals generated by the warp.
FIGS. 8(a)-(b) show the essential structure of a conventional sensing element. FIG. 8(a) is a plan view, and FIG. 8(b) is a sectional view cut along line Bxe2x80x94B in FIG. 8(a).
In FIGS. 8(a)-(b), the oxide film 2 at the bottom of the sensing element 103, which is arranged at the center of the silicon layer 3, is removed in order to allow weight portions 110a, 110b of the sensing element 103 to move freely. The sensing element 103 comprises four beams 111a, 111b, 111c, 111d, with semiconductor strain gauges 113a, 113b, 113c, 113d; the weight portions 110a, 110b with holes 15 for etching the oxide film as a sacrifice layer; and four beams 111e, 111f, 111g, 111h that support the weight portions 110a, 110b and have no semiconductor strain gauge. The weight portions 110a, 110b deform the eight beams. The semiconductor strain gauges 113a, 113b, 113c, 113d sense the deformations of the four beams 111a, 111b, 111c, 111d with the semiconductor strain gauges, and convert the deformations into electric signals. As shown in FIGS. 8(a)-(b), the sensing element 103 is composed of the silicon layer 3 having the holes 15, and the sensing element 103 sticks on the silicon substrate 1 through the oxide film 2. The sensing element 103 is supported at a position where it sticks on the silicon substrate 1 (the position is not shown in FIG. 8(a)).
FIG. 9 is a circuit diagram showing the semiconductor physical quantity sensor. An analog amplifier circuit 105 amplifies an output voltage of a Wheatstone bridge, which is composed of the four semiconductor strain gauges 113a, 113b, 113c, 113d. The digital adjustment circuit 104 adjusts the sensitivity and the temperature characteristics.
A description will now be given of the operation of an acceleration sensor, which is an example of the semiconductor physical quantity sensor. If a force generated by the vertical acceleration is applied to the semiconductor physical quantity sensor, a compressive stress acts on the two semiconductor strain gauges 113b, 113d of the four semiconductor strain gauges 113a, 113b, 113c, 113d to decrease their resistance. On the other hand, a tensile stress acts on the two semiconductor strain gauges 113a, 113c to increase their resistance. The change in the resistance causes the Wheatstone bridge circuit to output a sensor signal corresponding to the acceleration. Vcc indicates a high potential of a power supply voltage; GND indicates a grand potential; and V+ and Vxe2x88x92 indicate a positive potential and a negative potential, respectively.
FIGS. 10 and 11 are sectional views showing steps A-F in order in a conventional method for manufacturing the semiconductor physical quantity sensor.
At the step A, an insulating layer of the oxide film 2 such as BPSG film or PSG film is formed on the silicon substrate 1, and the silicon layer 3 made of polysilicon or the like is formed on the oxide film 2 to thereby construct a SOI substrate 100. Although not illustrated in the drawings, the previously-mentioned semiconductor strain gauges, the analog amplifier circuit 105, the digital adjustment circuit 104, the input/output terminal 106, the digital adjustment terminal 107, or the like are formed in the silicon layer 3.
At the step B, a resist film 4 is coated and patterned on the silicon layer 3. Then, a number of holes 15 are formed in the silicon layer 3 by wet etching using mixed acid of hydrofluoric acid (HF) or by dry etching using mixed gas of nitric acid (HNO3), and sulfur hexafluoride (SF6) and oxygen (O2), thus forming the sensing element 103 (indicated by an arrow). The sensing element 103 is formed in the silicon layer 3 including the weight portions 110.
At the step C, the oxide film 2, which is the sacrifice layer opposite to the bottom of the silicon layer 3, is removed by an etching liquid 5 such as HF.
At the step D, the sensing element 103 is cleaned by a displacement liquid 6 such as pure water and isopropyl alcohol (IPA), and then the displacement liquid 6 is vaporized to dry the sensing element 103. In the drying process, a surface tension of the displacement liquid 6 generates a suction force 7 toward the silicon substrate 1.
At the step E, the weight portions 110 of the sensing element 103 formed in the silicon layer 3 made of polysilicon with low rigidity are sucked and stuck on the silicon substrate 1 by the suction force 7. This is called a sticking phenomenon.
At the step F, the resist film is ashen and removed while the weight portions 110 stick on the silicon substrate 1.
If the weight portions 110 stick on the silicon substrate 1 in the sticking phenomenon, the physical quantity sensor is useless.
A description will now be given of a manufacturing method that prevents the sticking phenomenon (Japanese Patent Publication No. 7-505743).
FIGS. 12 and 13 are sectional views showing steps A-F in order in a conventional method for manufacturing the semiconductor physical quantity sensor. This method is disclosed in Japanese Patent Publication No. 7-505743.
At the step A, a sacrifice layer of an oxide film 2 such as BPSG and PSG is formed on a silicon substrate 1, and a silicon layer 3 made of polysilicon is formed on the oxide film 2.
At the step B, a resist film 4 is coated and patterned on the silicon layer 3, and a sensing element 103 is formed in the silicon layer 3.
At the step C, an etching liquid 5 etches the oxide film 2 in such a manner as to partially remain that the oxide film 2 as the sacrifice layer just below the silicon layer 3. The silicon layer 3 sticks on the silicon substrate 1 through the remaining oxide film 2.
At the step D, a photosensitive polymer 15 is coated and patterned in such a manner as to fill up a part A, from which the oxide film 2 as the sacrifice layer between the silicon layer 3 and the silicon substrate 1 has already been removed.
At the step E, an etching liquid 13 etches the remaining oxide film 2 in order to remove the oxide film 2 from a part B.
At the step F, the etching liquid 13 at the part where the oxide film 2 has already been removed is substituted with a displacement liquid 6 to dry the part B. At this time, a surface tension of the displacement liquid 6 causes a suction force 27 to act on the silicon substrate 1 as indicated by an arrow. This does not result in the sticking phenomenon in which the weight portions 100 of the sensing element 103 stick on the silicon substrate 1 at a position 30 inside the circle, because the photosensitive polymer 15 has a high rigidity.
FIG. 13(c) shows the dried sensing element 103. The weight portions 110 never stick on the silicon substrate 1 at the step G.
At the step H the photosensitive polymer 15 and the resist film 4 are removed in the drying process such as ashing, thereby manufacturing the sensing element 103 with movable weight portions 100.
There is another method for manufacturing a sensing element, and this will now be described.
FIGS. 14 and 15 are sectional views showing steps A-F in order in another conventional method for manufacturing a semiconductor physical quantity sensor. This method is disclosed in Japanese Patent Provisional Publication Nos. 7-209105 and 7-245414.
The steps A-C are the same as the steps A-C in FIG. 10.
At the step D, an etching liquid 5 is substituted with a displacement liquid (not illustrated), and a sublimation substance 30 such as palladichrolobenzene and naphthalene is liquidated. The displacement liquid is substituted with the sublimation substance 30 in such a manner that the sublimation substance 30 can fill up a space between the silicon substrate 1 and the silicon layer 3. Then, the sublimation substance 30 is fixed.
At the step E, the sublimation substance 30 is sublimed.
At the step F, the resist film 4 is removed in the drying process such as ashing, thereby manufacturing a sensing element 103.
If the photosensitive polymer 15 holds the sensing element 103 during the drying, it is difficult to uniformly fill up the space between the silicon layer 3 and the silicon substrate 1 with the photosensitive polymer 15 because the patterning accuracy is deteriorated by an unevenness of several xcexcm on the surface of the photosensitive polymer 15. Moreover, if the photosensitive polymer 15 filled between the silicon layer 3 and the silicon substrate 1 is not completely removed in the drying process, a residue 31 stays behind. This lowers the percentage of non-defective articles and increases the manufacturing cost.
If the photosensitive polymer 15 stays behind between the silicon layer 3 and the silicon substrate 1, the movable range of the weight portions 110 of the sensing element 103 is narrowed, and this deteriorates the sensing accuracy and reliability. Moreover, there is the necessity for etching the oxide film 2 of the sacrifice layer twice and patterning the photosensitive polymer 15, and this increases the manufacturing cost.
If the sublimation substance 30 holds the sensing element during the drying, the sublimation substance 30 cannot be removed completely. Therefore, an alien substance 32 mixed in the sublimation substance 30 may remain. This deteriorates the sensing accuracy and reliability.
In view of the foregoing, it is an object of the present invention to prevent the sticking phenomenon and improve the sensing accuracy and reliability.
The invention accomplishes the above object by providing a physical quantity sensor manufacturing method, which uses a SOI substrate composed of a silicon substrate as a first layer, an insulating layer as a second sacrifice layer formed on the first layer, and a silicon or polysilicon layer as a third layer formed on the second layer, and which comprises machining the third layer to form a sensing element for sensing physical quantities and removes the second layer to form a semiconductor physical quantity sensor, the method comprising the steps of coating the third layer with protective film; forming a sensing element by using the protective film as a mask; removing the second sacrifice layer by wet etching; substituting an etching liquid used in the wet etching with a displacement liquid; drying the sensing element by removing the displacement liquid sticking on the sensing element in a state wherein an acceleration in an opposite direction to a direction toward the first layer is applied to the sensing element; and removing the protective film by dry etching.
An acceleration to be applied to the sensing element is preferably obtained by a centrifugal force generated by revolving the SOI substrate.
The sensing element is preferably dried so that a suction force of the sensing element against the first layer due to a surface tension of the displacement liquid, the centrifugal force (FS) and a spring force FK of the sensing element can satisfy the following condition: (Fr+FK) greater than FS.
Thus, the acceleration in a direction opposite to the direction of the spring force generated by revolving the SOI substrate and the spring force generated at the base of the weight portions of the sensing element exceed the suction force generated by the surface tension of the displacement liquid, and this prevents the sticking phenomenon.
Preferably, the displacement liquid is stuck on the sensing element until the acceleration reaches a predetermined value, and the displacement liquid is removed and the sensing element is dried after the acceleration reaches the predetermined value.
This prevents the sticking phenomenon during the low-speed revolution.
The sensing element is preferably dried by an inert gas.
Preferably, the first layer is electrically connected with the second layer, and the SOI substrate is revolved.
This prevents the sticking phenomenon since the static electricity generated between the wafer and the surrounding air during the revolution is canceled by minus ions or is shifted toward the first layer.
The displacement liquid is preferably vapor, pure water or isopropyl alcohol (IPA).
A manufacturing apparatus for use in the above-mentioned semiconductor physical quantity sensor manufacturing method preferably comprise at least a rotary shaft, a drying tank and a support plate for fixing the SOI substrate, and at least either one of the rotary shaft and the drying tank has holes for spraying the displacement liquid.